Conformational behaviour of highly hindered ethanes: meso- and racemic-2,2,2′,2′-tetramethyl-1,1′-biindanyl

Abstract

MM3(94), AM1, and STO-3G calculations on racemic 2,2,2′,2′-tetramethyl-1,1′-biindanyl (1) show that seven minima are present on its conformational potential energy surface. 1 crystallizes in the space group Pbcn (no. 60) with cell parameters a = 11.190(7), b = 11.679(7), c = 13.19(1) Å, V = 1724(3) Å3, Z = 4, R = 0.0357, Rw = 0.0362 in the conformation predicted to be the global minimum by STO-3G and AM1, but not by MM3(94). Application of the Karplus equation and observation of a deshielded proton signal indicate that the same conformation is the major species present in solution, perhaps along with another conformation. The meso isomer 2 shows evidence of slow exchange between conformers in its room temperature NMR spectra. Barriers to exchange were obtained from 13C NMR coalescence experiments and from 1H NMR line shape analysis of the exchanging AB pattern. The potential energy surface for 2 was calculated as for 1 and used to determine the nature of the process observed. The slow exchange was calculated to be between two enantiomeric conformers. The populated conformers for both 1 and 2 had the benzene rings from the two halves of the dimer stacked in an offset manner and the favourable interactions associated with this stacking may be a factor in the relative stabilities of these conformations. Key words: MM3, molecular mechanics, hindered rotation, dialkyldiphenylethanes, X-ray crystal structure.